Subterranean drilling systems that employ downhole drilling motors are commonly used for drilling boreholes in the earth for oil and gas exploration. FIG. 1 is a schematic isometric, partial, cut-away view of a prior art subterranean drilling system 100. The subterranean drilling system 100 includes a housing 102 enclosing a downhole drilling motor 104 (i.e., a motor, turbine, or any other device capable of rotating a shaft) that is operably connected to an output shaft 106. A thrust-bearing apparatus 108 is also operably coupled to the downhole drilling motor 104. A rotary drill bit 112 configured to engage a subterranean formation and drill a borehole is connected to the output shaft 106. The rotary drill bit 112 is shown as a “roller cone” type bit including a plurality of roller cones 114. However, other types of rotary drill bits, such as so called “fixed cutter” drill bits are also commonly used. As the borehole is drilled, pipe sections may be connected to the subterranean drilling system 100 to form a drill string capable of progressively drilling the borehole to a greater depth within the earth.
The thrust-bearing apparatus 108 includes a stator 116 that does not rotate and a rotor 118 that is attached to the output shaft 106 and rotates with the output shaft 106. The stator 116 and rotor 118 each include a plurality of bearing elements 120 that may be fabricated from polycrystalline-diamond compacts that provide diamond bearing surfaces that bear against each other during use.
In operation, high pressure drilling fluid is circulated through the drill string and power section (not shown) of the downhole drilling motor 104, usually prior to the rotary drill bit 112 engaging the bottom of the borehole, to generate torque and rotate the output shaft 106 and the rotary drill bit 112 attached to the output shaft 106. Unless rotated from above by the drill rig rotary, the housing 102 of the downhole drilling motor 104 remains stationary as the output shaft 106 rotates the rotary drill bit 112. When the rotary drill bit 112 engages the bottom of the borehole, a thrust load is generated, which is commonly referred to as “on-bottom thrust” that tends to compress the thrust-bearing apparatus 108. The on-bottom thrust is carried, at least in part, by the thrust-bearing apparatus 108. Fluid flow through the power section may cause what is commonly referred to as “off-bottom thrust,” which is carried, at least in part, by another thrust-bearing apparatus that is not illustrated. The drilling fluid used to generate the torque for rotating the rotary drill bit 112 exits openings formed in the rotary drill bit 112 and returns to the surface, carrying the cuttings of the subterranean formation through an annular space between the drilled borehole and the subterranean drilling system 100. Typically, a portion of the drilling fluid is diverted by the downhole drilling motor 104 to cool and lubricate both the thrust-bearing apparatus 108 and the other thrust-bearing apparatus. Lateral loads may also be applied to the subterranean drilling system 100 during drilling of the borehole.
Both the off-bottom and on-bottom thrust carried by the thrust-bearing apparatuses can be extremely large. Accordingly, the operational lifetime of the thrust-bearing apparatuses often determines the useful life for the subterranean drilling system 100. For example, during operation, bending and/or side loading of the subterranean drilling system 100 can cause some of the bearing elements 120 of the thrust-bearing apparatus 108 to experience higher than desired stresses and/or temperatures. A slight angular misalignment between the stator 116 and rotor 118 of the thrust-bearing apparatus 108 due to such bending and/or side loading may cause certain bearing elements 120 of the thrust-bearing apparatus 108 to partially contact or even fail to contact each other during operation, causing other bearing elements to contact each other at higher than desired stresses. The bearing elements 120 of the thrust-bearing apparatus 108 that remain in contact with each other may experience sufficiently large stresses and sufficiently high temperatures during drilling to damage or even fracture and, ultimately, may cause premature failure of the subterranean drilling system 100.
Therefore, manufacturers and users of subterranean drilling systems continue to seek improved thrust-bearing apparatuses and radial-bearing apparatuses that can accommodate such loading and/or misalignment between bearing components.